RGB values -- how to match (not percentile)

[coningham]

There are many methods for getting the result you want.

If you want to use the RGB value (which is only the value of one pixel or the average for a few pixels), you'll need to paint over the image using the Paint Brush with the blend mode set to "Color".

Another method is to select the portion that you want to modify (if you pasted it in a new layer the work will be even easier) and apply the available photo editing adjustments.
On the image I posted, for example, I used (using version X, btw) Red/Green/Blue, Hue/Saturation/Lightness and Curves. Not that all 3 were really necessary, but this gives you an idea of what you can use.

Another thing is to soften the edges from the pasted portion so that it blends even better with the rest. In this case, you could use the Eraser tool set at a low opacity on the neck.

This is really a fun process, but a lot of experimentation is involved, so don't be afraid to try new tools